Question

应该非常简单：我有一个InputStream，我想偷看（不读取）前两个字节，即我希望在偷看之后，InputStream的“当前位置”为0。最好和最安全的方法是什么？

回答 - 正如我所怀疑的，解决方法是将其包装在提供可标记性的BufferedInputStream中。谢谢Rasmus。

Answer 1

对于一般的InputStream，我会将它包装在BufferedInputStream中并执行以下操作：

BufferedInputStream bis = new BufferedInputStream(inputStream);
bis.mark(2);
int byte1 = bis.read();
int byte2 = bis.read();
bis.reset();
// note: you must continue using the BufferedInputStream instead of the inputStream

Answer 2

您可能会发现PushbackInputStream非常有用：

http://docs.oracle.com/javase/6/docs/api/java/io/PushbackInputStream.html

Answer 3

当使用BufferedInputStream时，确保inputStream尚未缓冲，双缓冲将导致一些人很难找到错误。此外，您需要以不同方式处理读者，转换为StreamReader，如果Reader是缓冲的，缓冲将导致字节丢失。此外，如果您使用的是Reader，请记住您没有读取字节，而是使用默认编码中的字符（除非设置了显式编码）。您可能不知道的缓冲输入流的示例是URL URL; url.openStream（）;

我没有任何关于此信息的参考，它来自调试代码。我遇到问题的主要案例是从文件读入压缩流的代码。如果我在开始调试代码时记得正确，那么Java源代码中的注释表明某些事情总是无法正常工作。我不记得使用BufferedReader和BufferedInputStream的信息在哪里来自但我认为即使是最简单的测试也会立刻失败。记住测试一下，你需要标记超过缓冲区大小（BufferedReader与BufferedInputStream不同），当读取的字节到达缓冲区末尾时会出现问题。请注意，源代码缓冲区大小可能与您在构造函数中设置的缓冲区大小不同。我这样做了一段时间，所以我对细节的回忆可能有些偏差。使用FilterReader / FilterInputStream进行测试，将一个添加到直接流，一个添加到缓冲流以查看差异。

Answer 4

我在这里找到了一个PeekableInputStream的实现：

http://www.heatonresearch.com/articles/147/page2.html

本文中显示的实现的想法是它在内部保留一系列“peeked”值。调用read时，首先从peeked数组返回值，然后从输入流返回值。当您调用peek时，将读取值并将其存储在“peeked”数组中。

由于示例代码的许可证是LGPL，因此可以附加到此帖子：

package com.heatonresearch.httprecipes.html;

import java.io.*;

/**
 * The Heaton Research Spider Copyright 2007 by Heaton
 * Research, Inc.
 * 
 * HTTP Programming Recipes for Java ISBN: 0-9773206-6-9
 * http://www.heatonresearch.com/articles/series/16/
 * 
 * PeekableInputStream: This is a special input stream that
 * allows the program to peek one or more characters ahead
 * in the file.
 * 
 * This class is released under the:
 * GNU Lesser General Public License (LGPL)
 * http://www.gnu.org/copyleft/lesser.html
 * 
 * @author Jeff Heaton
 * @version 1.1
 */
public class PeekableInputStream extends InputStream
{

  /**
   * The underlying stream.
   */
  private InputStream stream;

  /**
   * Bytes that have been peeked at.
   */
  private byte peekBytes[];

  /**
   * How many bytes have been peeked at.
   */
  private int peekLength;

  /**
   * The constructor accepts an InputStream to setup the
   * object.
   * 
   * @param is
   *          The InputStream to parse.
   */
  public PeekableInputStream(InputStream is)
  {
    this.stream = is;
    this.peekBytes = new byte[10];
    this.peekLength = 0;
  }

  /**
   * Peek at the next character from the stream.
   * 
   * @return The next character.
   * @throws IOException
   *           If an I/O exception occurs.
   */
  public int peek() throws IOException
  {
    return peek(0);
  }

  /**
   * Peek at a specified depth.
   * 
   * @param depth
   *          The depth to check.
   * @return The character peeked at.
   * @throws IOException
   *           If an I/O exception occurs.
   */
  public int peek(int depth) throws IOException
  {
    // does the size of the peek buffer need to be extended?
    if (this.peekBytes.length <= depth)
    {
      byte temp[] = new byte[depth + 10];
      for (int i = 0; i < this.peekBytes.length; i++)
      {
        temp[i] = this.peekBytes[i];
      }
      this.peekBytes = temp;
    }

    // does more data need to be read?
    if (depth >= this.peekLength)
    {
      int offset = this.peekLength;
      int length = (depth - this.peekLength) + 1;
      int lengthRead = this.stream.read(this.peekBytes, offset, length);

      if (lengthRead == -1)
      {
        return -1;
      }

      this.peekLength = depth + 1;
    }

    return this.peekBytes[depth];
  }

  /*
   * Read a single byte from the stream. @throws IOException
   * If an I/O exception occurs. @return The character that
   * was read from the stream.
   */
  @Override
  public int read() throws IOException
  {
    if (this.peekLength == 0)
    {
      return this.stream.read();
    }

    int result = this.peekBytes[0];
    this.peekLength--;
    for (int i = 0; i < this.peekLength; i++)
    {
      this.peekBytes[i] = this.peekBytes[i + 1];
    }

    return result;
  }

}

如何查看InputStream中的前两个字节？

4 个答案: